This invention relates generally to techniques used in geophysical well logging, and more particularly to new techniques for automatically processing dipmeter signals or displacement measurements obtained between these signals to produce more accurate dip and aximuth representations of subsurface formations.
A common method of measuring the dip angle and direction or azimith of subsurface formations employs a dipmeter tool passed through a borehole drilled into the subsurface formations. This tool may apply any of numerous means to obtain geophysical signals representative of variations of a particular formation characteristic, such as its resistivity. One such tool is described in the paper: "The High Resolution Dipmeter Tool", by L. A. Allaud and J. Ringot, published in the May-June 1969 issue of The Log Analyst.
Dip and azimuth measurements representing the inclination of a formation characteristic or feature may be determined from dipmeter signals containing information representing the intersection of such a feature at three or more radially spaced points on the borehole surface. The displacement between two points intersecting a common feature may be determined, under favorable circumstances, by correlating pairs of the dipmeter signals, each having a similar response to the common feature. Two displacements between three related points determine the position of a plane. The position of the plane is conveniently expressed by its dip .theta., an angle measured from a reference (usually horizontal) plane and its azimuth .PHI., an angle measured from a reference direction (usually true North).
Typically, the dipmeter signals are recorded on computer compatible magnetic tape at the well site for later processing. The recorded signals are processed using any of several techniques. Manual, semi-automatic and fully automatic processing may be used with the automatic processing being performed with either analog or digital computers. When digital computers are used, a computer program is also required.
A computer program to perform the digital processing operations is described in a paper, "Automatic Computation of Dipmeter Logs Digitally Recorded on Magnetic Tape" by J. H. Moran, et al and published in the July, 1962 issue of the Journal of Petroleum Technology. An additional computer program is described in the paper, "Computer Methods of Diplog Correlation" by L. G. Schoonover, et al, pages 31-38, published in the February 1973 issue of Society of Petroleum Engineers Journal. Further, programs to process digitally-taped dipmeter data may be obtained from digital computer manufacturers, such as IBM.
Results from digital processing are normally presented in tabular listings as dip and azimuth measurements versus borehole depth. When desired, the individual displacements found between the correlated curve pairs which led to the dip and azimuth values may also be presented. Further, most such programs will provide the ability to vary both the length of the correlation interval and the step used to move this interval between each correlation sequence. For the next sequence, the same correlation length is used, but the actual interval correlated is moved by one correlation step length.
At each step or depth level, one sequence of displacements between various pairs of signal combinations may be obtained. A typical sequence includes at least two displacements but may include a round of up to six displacements in each sequence when four separate signals are employed, for example. When a round of more than two displacements in one sequence is obtained, the displacements may be combined into many more possibly different combinations, each combination corresponding to perhaps a different dip and azimuth measurement. Since only two related displacements are required, it is common practice to utilize only what appears to be the two best qualified displacements. All others are discarded without further consideration, thereby producing only one result per sequence. Further, little is retained as to the accuracy or quality of either the discarded or the utilized displacements.
When large numbers of measurements result, as from recent high resolution dipmeter techniques, tabular listings are usually augmented by graphic presentations of dip and aazimuth representations. The graphic displays vary with the interpretation objective, depending upon whether the purpose is for stratigraphic or structural studies. Accordingly, relationships between the corresponding dip and azimuth measurements and their continuity with depth are considered in different manners.
Graphic displays used in stratigraphic analysis are typically the azimuth frequency plot (no dip or depth representation) and the Schmidt net and the Stereonet (azimuth versus dip but still no depth representation). These nets and several variations thereof have known statistical characteristics in that they may enhance either low or high dip measurement point groupings. Note that in their use, the dip and azimuth value for each measurement is combined and represented by a point in these nets. A description of some of these displays and their application is given in the paper "Stratigraphic Applications of Dipmeter Data in Mid-Continent" by R. L. Campbell, Jr., published in September 1968 in the American Association of Petroleum Geologists Bulletin.
Stratigraphic and structural analyses distinguish themselves in the type of information needed. In stratigraphic analysis, the dipmeter signals hopefully represent bedding planes within the boundaries of a given geological unit. These bedding planes have little, if any, regional extent. In structural analysis, a deliberate attempt may be made to mask out such sedimentary features in favor of enhancing the boundaries of the individual strata.
Short lengths (1 to 2 or 3 feet) of dipmeter signals are correlated to obtain stratigraphic information while long lengths (10 to 20 or 30 feet) of signals are often correlated to obtain structural information. While use of long correlation lengths to obtain structrual dip has been standard practice for some time, there are certain disadvantages associated with this practice. One is that the use of long correlation lengths masks dip patterns needed for stratigraphic analysis, thus additional computations must be made using a short length to obtain stratigraphic information. Another is that most long correlation length techniques may be influenced by frequently occurring stratigraphic features having a common dip and direction, even though each such feature is less pronounced than the structural feature. Thus, the use of long correlation lengths does not assure obtaining accurate structural dip information. Yet another disadvantage is that such correlation techniques tend to ignore possibly objectionable effects of rotation of the dipmeter tool within the long correlation interval.
The preferred approach is to obtain the detailed information available only from short signal intervals and then apply previously mentioned trend analysis to separate the stratigraphic and structural dips. However, as the correlation interval is shortened, the probability of obtaining a completely erroneous displacement increases substantially. The wrong peak on the correlation function produced in the correlation process may be used to determine the displacement. Such invalid displacements may be combined with valid displacements and produce an erroneous dip which add scatter and confuse valid trends or when systematically erroneous, may even appear as false trends.
As a compromise, longer correlation intervals than are actually desired are employed to artificially reduce this scatter to an acceptable level so that any valid trend which may be present might be found.
It is therefore an object of this invention to provide a technique to reduce the scatter in dip and azimuth measurements used in determining structural dip.
One technique which is employed to reduce scatter and find structural dip is to average long intervals of dip measurements obtained from much shorter intervals. Unfortunately, valid structural trends present only for short intervals may be masked completely by such an averaging process. Further, the resolution, quality and correlogram peak position obtained by correlating short intervals tends to vary considerably; consequently, the corresponding displacements may lack accuracy. Certain combinations of such displacements may compound the variation and introduce unacceptable inaccuracies in the resulting dip and azimuth measurements.
It is therefore an additional object of the present invention to provide a technique to improve the accuracy and reduce the scatter of dip and azimuth measurements without necessitating long interval averaging.
Some of the averaging techniques include a preliminary process of sorting or discarding apparently stray dips before averaging to prevent their contributing to the average. This process adds both time delays and expense to a process which already produces too few dips for many purposes. Further, some of the apparent strays may actually be part of a valid trend which was unfortunately just sampled infrequently. Both the discarding and averaging processes suppress such valid dips. Still further, the apparently stray dip may have been produced by combining a valid displacement with an invalid displacement. Unfortunately, discarding this dip also discards the valid displacement information.
It is therefore a further object of the present invention to provide a technique to minimize the likelihood of discarding valid displacement information combined with invalid or inaccurate displacements.
As previously discussed, there are prior art techniques for statistically analyzing either the dip or azimuth information for long interval trends. These methods usually employ polar chart representations to classify the dip and/or azimuth measurements. In these plots, the dip varies with distance from either the center or the edge of the plots and the azimuth varies with the radial distribution from the center of the plot.
However, when one considers the type of errors likely to take place in the correlation processes, particularly in deviated holes, it is desirable that any analysis not separate the dip from the azimuth values for the purposes of the analysis. The analysis should be able to detect any interrelationship between the dip and azimuth for the individual measurements. More particularly, the analysis should respect the fact that an erroneous displacement can be concealed when combined with another displacement and expressed as a dip and azimuth measurement.
It is therefore a further object of the present invention to provide a technique for analyzing displacements rather than combinations of displacements or the resulting dip or azimuth measurements.
Prior art methods do attempt to select only the best displacements or combinations thereof by assigning a quality rating according to the correlation process which determined the displacement. The best rated displacements are selected while discarding poor quality displacements. The best rated displacements may be distorted or exaggerated due to failure of the signal source to maintain its proper position in the borehole, while poorer rated displacements may be obtained from sources in a much better position to produce more accurate displacements.
It is therefore a particular object of the present invention to provide a technique to retain even apparently poor quality or less accurate displacements, as they may in fact be valid, until a better basis for judging the validity of these displacements is available, thereby preventing premature loss of this information.
There are numerous methods of obtaining displacement measurements between pairs of geophysical signals. It is well known how to use one of several different correlation functions to produce a correlogram--a function representing the correlation factor, likeness or similarity of signal features in given intervals of two signals versus the displacement measured between the intervals. The displacement corresponding to the best correlation or likeness is usually selected as the displacement measurement and the corresponding correlation factor or likeness used to express the quality. Also, the shape of the correlogram adjacent to this best correlation factor is related to the displacement measurement accuracy.
Another correlation method of obtaining displacements recognizes characteristic signal features by their patterns and determines which features on both pairs of signals correspond to one another by comparing those characteristics. Each comparison yields a quality factor; the best comparing pattern determines the feature correspondence and displacement, and the nature of a characteristic of the pattern (for example, the rate of change in signal amplitude) provides a measure of displacement accuracy. Thus, with a variety of techniques available to determine displacement measurements between pairs of signals, the corresponding quality factors and some measure of displacement accuracy, it is desirable to have a general technique to process these displacements which is relatively independent of the technique used to obtain these measurements.
It is therefore an object of the present invention to provide a general technique to process displacement measurements to determine the dip of a formation and, particularly, to provide a technique which utilizes to advantage the quality factor and displacement accuracy information corresponding to each such displacement when available.
in accordance with these and other objects of the present invention, apparatus and methods are provided which automatically determine with a machine the dip of a formation feature reflected on geophysical signals derived from signal sources located at different positions in a borehole penetrating subsurface formations. The dip is determined by processing displacements obtained between pairs of these signals. These displacements may be obtained by comparing similarities of the signal features in a given interval of the signals. Each displacement is represented by its possible dips. At least two displacements obtained between different intervals of at least one pair of geophysical signals are so represented. The position of coincidence between these possible dip representations is determined along with the corresponding dip of a formation feature.
In one embodiment of the invention, the possible dip representations correspond to a line projected in a plane normal to the borehole or, if the axis of the borehole varies substantially over the intervals considered, the mean axis of the borehole. The orientation of the line is determined from the orientation of the signal sources in the borehole. The distance between the line and the intersection point of the borehole axis with the plane represents the range of possible dip values for the displacement and depends upon the magnitude of the displacement. When two different displacements corresponding to the same formation but measured with different orientations relative to features of the formation are so represented, their line representations intersect. The intersection point corresponds to the actual dip of the formation and its orientation corresponds to the azimuth of this dip. Displacements between various signal pairs within a particular interval and within at least one additional nearby interval are represented by such lines. When several intersections at different positions occur, the position of the highest coincidence of intersections is determined as corresponding to a more accurate formation dip than might be determined by combining only two displacements.
In one aspect of the invention, a quality is associated with each displacement and used to control the intensity or weight of the line representation. For a given line width, the highest intensity line corresponds to the best quality displacement. Therefore, the intensity or weight of the line intersections will vary not only with the number of lines intersecting at the same position, but also with the quality associated with displacements represented by these lines. The position of the most intense intersection determines the most accurate formation dip.
In another aspect of the invention, a displacement accuracy or error factor is associated with each displacement and used to control the line width accordingly. Wider lines represent less accurate displacements while narrower lines represent more accurate displacements. The intensity of the lines representing the same quality factor is decreased for the wider lines and increased for the narrower lines; i.e., the intensity is inversely proportional to the line width. In this embodiment, the intensity of the line intersections varies not only with the number of coincident intersections but also with both the accuracy and quality of the displacements they represent. Again, the most intense intersection corresponds to the most accurate dip.
Since all the displacements which may be obtained at a given depth level between the various pair combinations of two or more signals may be represented as well as the displacements from nearby depth levels, a large number of displacements may be represented in the same plane. All the displacements corresponding to the same formation dip intersect at substantially the same position. Therefore, the position corresponding to the highest coincidence of line intersections most accurately represents the dip of this formation.
Further, since invalid and inaccurate displacements will not consistently intersect at the same position as the valid displacements, no penalty is imposed by representing all displacements regardless of their apparent quality or accuracy. Therefore, there is not need to prematurely and perhaps, somewhat arbitrarily, pass judgment on each displacement in order to determine only the two best rated displacements required to determine the formation dip in the prior art techniques. Rather, all displacements are processed without regard to apparent quality or accuracy except as mentioned above to vary the line width and intensity. Furher, the formation dip may be determined and confirmed by substantially more than two displacements. Still further, these displacements may have been obtained between unrelated pairs of signals or between the same pair of signals but at different nearby intervals or depth levels.
In highly deviated holes characteristically employed to exploit offshore oil and gas producing formations, the ability to obtain accurate formation dips from apparently inaccurate or poor quality displacement measurements is an important advantage since signal measurement problems associated with deviated boreholes often contribute to inaccuracies and misleading quality factors for displacement measurements between signals obtained in different sectors of the borehole. For example, the signal obtained from the signal source in good contact with the borehole wall may not contain the same signal features as a signal obtained from a signal source not in contact with the borehole wall, because of mechanical problems associated with maintaining the desired source position in such highly deviated holes.
Displacements associated with poorly positioned signal sources are often distorted. Further, it is difficult to detect when such distortion occurs. For example, the quality factors associated with such distorted displacements are often of the best quality factors in a particular interval. However, in accordance with the features of the present invention, the displacement representations of such distorted displacements will not intersect at substantially the same position; i.e., they are scattered in accordance with their varying distortion, and accordingly, do not coincide with the intersections of valid displacements.
A further advantage of the present invention occurs when only two signal sources contain reliable signal features, the remaining sources suffering from signal distortion or attenuation problems. In such cases, the prior art techniques requiring two displacements over corresponding intervals on three different signals must attempt to utilize one of the bad signals in order to produce any results whatsoever. However, in utilizing the present invention, it is possible to obtain useful results under certain circumstances with only two signals.
In addition to the above advantages, the techniques of the present invention may be used to preserve the displacement integrity information for consideration in the displacement processing in a manner which enhances the determination of the dip corresponding to the more accurate and better quality displacements. This is carried out by varying the effect of each displacement representation in accordance with the associated quality factor and/or displacement accuracy. For a given displacement representation, the weight the displacement contributes to the dip determination is increased for the higher integrity displacements, while decreased for the lower integrity displacements. This allows each displacement to possibly contribute in some degree to the dip determination.
For a better understanding of the present invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawings, the scope of the invention being pointed out in the appended claims.